1. Field of the Invention
The present invention relates to a dielectric filter and dielectric duplexer obtained by forming conductive films on the inside and outside of a dielectric block, and relates to a communication device using the same.
2. Description of the Related Art
Dielectric filters formed by providing a plurality of resonant transmission lines obtained by forming conductive films on the inside and outside of a dielectric block are used in communication devices such as cellular phones.
Conventional dielectric filters using the dielectric block and in which an attenuation pole is generated near a passband are disclosed in Japanese Unexamined Patent Publication No. 5-145302.
A dielectric filter disclosed in the above application has a multipath construction in which two internal conductors disposed in a dielectric block and provided with a plurality of internal-conductor-formed holes, each of which constitutes a resonant transmission line, are capacitively coupled with an input/output electrode formed on the external face of the dielectric block.
The above multipath construction produces an attenuation pole on the low-frequency side or the high-frequency side of a passband. In order to dispose this attenuation pole close to the passband, leap coupling between the second resonant transmission line from the endmost one among the plurality of internal-conductor-formed holes arranged in the conductive block and the input/output electrode should be increased.
However, when the capacitance between the second resonant transmission line from the endmost one and the input/output electrode is increased, since the capacitance (the amount of external coupling) between the first (endmost) resonant transmission line and the input/output electrode is relatively decreased, the characteristics of the passband, in particular a reflection characteristic, are worsened.
When the first resonant transmission line and the input/output electrode are disposed close to each other in order to increase the external coupling therebetween, a problem arises in that since the electric field intensity is increased, the insertion loss is worsened.
Accordingly, objects of the present invention are to provide a dielectric filter and dielectric duplexer in which the attenuation pole is disposed close to the passband without degrading the characteristics of the passband and without increasing the insertion loss and to provide a communication device provided therewith.
To this end, according to a first aspect of the present invention, there is provided a dielectric filter including a dielectric block having substantially a rectangular parallelepiped shape, a plurality of internal-conductor-formed holes provided substantially parallel to one another in the dielectric block, each including an internal conductor formed on the internal surface thereof, an external conductor formed on the external surface of the dielectric block, and an input/output electrode formed on the external surface of the dielectric block. In the dielectric filter, the input/output electrode is formed from a side face, which is an end face of the dielectric block in the arrangement direction of the internal-conductor-formed holes, to a bottom face, which is a mounting face of the dielectric block facing a mounting substrate, a capacitance is generated between the internal conductor of a first internal-conductor-formed hole closest to the end face and the internal conductor of a second internal-conductor-formed hole in the neighborhood of the first internal-conductor-formed hole, and the cross section of at least the first internal-conductor-formed hole is a noncircular shape extending along the side face.
Since the electrical field intensification is reduced in an external coupling part between a resonant transmission line using the internal-conductor-formed hole closest to a side face of the dielectric block and the input/output electrode and mutual coupling between two resonant transmission lines using the internal-conductor-formed hole closest to the side face and the internal-conductor-formed hole next to it is increased, the attenuation pole can be disposed closer to the passband.
According to a second aspect of the present invention, a dielectric duplexer includes a dielectric filter according to the first aspect of the present invention. In the dielectric duplexer, the input/output electrode of the dielectric filter serves as one of a transmission signal input electrode, a reception signal output electrode, and an antenna connection electrode.
This aspect of the invention enables the dielectric duplexer to have a sufficient amount of attenuation between the transmission band and the reception band.
According to a third aspect of the present invention, a communication device includes the dielectric filter according to the first aspect of the present invention or the dielectric duplexer according to the second aspect of the present invention.
By employing a small band-pass filter having excellent in-band characteristics and having a sufficient amount of attenuation in the neighborhood of the passband, a miniaturized communication device having the excellent communication performance can be constructed.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.